The present invention relates generally to interplanetary (deep space) navigation of spacecraft and, more specifically, to a method and apparatus for autonomous, (without using ground-based systems) navigation using the sun.
Navigation plays a critical role in deep space missions since a spacecraft launched to a planet, asteroid, or comet must be accurately navigated to reach its destination in a close flyby with, orbit at, or landing on the targeted body. Current navigation systems for interplanetary missions include two types: the ground-based radio Doppler tracking and the onboard optical imaging. In almost every planetary mission to date, the spacecraft navigation has had to rely heavily on the global radio Doppler tracking system, which has 11 Deep Space Network (DSN) antenna dishes of sizes 70 m and 34 m, located in Goldstone, Calif., Canberra, Australia, and Madrid, Spain. The most fundamental radio navigation measurements are the two-way Doppler and range, acquired at S- and X-band frequencies by the DSN. The Doppler and range data are extracted from the incoming radio signals at the tracking stations on the ground and then processed by navigation personnel for estimating and predicting the spacecraft trajectory.
Since the orbit accuracy provided by the ground-based navigation system is insufficient for close encounters with planets or their satellites, an onboard optical navigation system has been developed and applied in many space missions. Optical images of the target planets or their satellites against star backgrounds were obtained through science imaging instruments on board the spacecraft. These optical images were transmitted through DSN to the ground and merged with the Doppler and range data to obtain an orbit determination solution of the spacecraft trajectory.
Autonomous spacecraft navigation is highly desirable for space missions, especially for missions that require updates of a spacecraft""s position in real time and that require frequent thrust and trajectory adjustments, such as missions that use solar electric propulsion or solar sails. Autonomous navigation can increase spacecraft""s capability and flexibility, taking immediate action in critical situations without any two-way light time delay, thereby increasing the spacecraft""s survivability and reducing risk. It also reduces mission operation costs, with daily manual operation being replaced by onboard autonomous operation.
Furthermore, due to the growing number of on-duty spacecraft plus those set to explore the solar system in the next decade, the DSN faces an inevitable communication over-crowding problem. New techniques and systems are becoming increasingly desirable to free up the DSN from time-consuming tasks. For this purpose and also, as noted above, for reducing operational costs in future space missions, the United States National Aeronautics and Space Administration (NASA) has called for the development of more efficient autonomous navigation systems. Autonomous navigation is not only a key element for achieving aggressive low-cost space missions, but also a prerequisite for space missions using solar electric propulsion or solar sails.
As an effort in this direction, a prototypical autonomous optical navigation system has been developed by the Jet Propulsion Laboratory (JPL) for testing in the Deep Space 1 (DS-1) mission. (See S. Bahaskaran, J. E. Riedel, and S. P. Synnott, xe2x80x9cAutonomous optical navigation for interplanetary missions,xe2x80x9d Space Sciencecraft Control and Tracking in the New Millennium, E. Kane Casani, M. A. Vander Does, Editors, Proc. SPIE vol. 2810, 1996, pp. 32-43; and J. E. Riedel, S. Bhaskaran, S. P. Synnott, W. E. Bollman, and G. W. Null, xe2x80x9cAn autonomous optical navigation and control system for interplanetary exploration missions,xe2x80x9d IAA paper IAA-L-0506, Second IAA International Conference on Low-cost Planetary Missions, Laurel, Md., April, 1996.) The technique of this optical navigation system is inherited from the Galileo mission. The main objectives of this prototype system are to test the autonomous optical image processing on board the spacecraft and to use the data to determine the spacecraft trajectory without the help from the controllers on earth.
Historically, this optical navigation system has only been used in approach phase. Whether it alone is sufficient to navigate the entire mission is still under study. One of the main limiting factors to the current optical navigation system is the lack of sufficient visible reference bodies with well-known ephemerides during the long cruise time. Bhaskaran et al at JPL indicated in their paper (referenced above) that asteroids would make more viable candidates for uses as beacons than planets due to their proximity and quantity. However, the knowledge of the heliocentric position of the beacon asteroid is required if the asteroid is to be used as the reference. Therefore, for the DS-1 mission that uses asteroids for beacons, a ground-based campaign has been conducted to improve the ephermerides of the beacon asteroids used for the mission. However, if the mission is to explore deeper space, accurate positions of the asteroids there are hardly known, and there will be no easy way to improve the ephermerides of the beacon asteroids before the mission. As a result, the current optical navigation system may not be suitable for cruise in the deeper space missions, such as missions to the outer solar system.
The autonomous solar navigation method and apparatus of the invention is a completely self-contained autonomous navigation system developed for interplanetary space missions. This navigation system uses only onboard observations of the Sun in combination with the onboard spacecraft attitude data to estimate and predict the spacecraft""s orbit autonomously. Unlike the current ground-based navigation system, which uses two-way coherent radio Doppler tracking through the Deep Space Network (DSN), the system of the invention does not need the DSN or any control from the ground. The self-containment allows the spacecraft to navigate independently without relying on signals transmitted from the Earth. In addition, since the Sun is an active energy source and visible everywhere in the universe, this navigation system is applicable to spacecraft moving anywhere in space.
As shown in FIG. 1, the spacecraft""s state (its position and velocity vector) is determined by processing the solar observation data on board. Two types of solar data can be used for estimating the state of the spacecraft:
1. The directional data that measures the change in the Sun""s direction as a function of time, viewing the Sun from the spacecraft against the star background; and
2. The optical Doppler shifts observed in sunlight that provide the line-of-sight velocity of the spacecraft relative to the Sun.
All of the six orbit elements that define a spacecraft""s orbit can be completely determined with measurements of the Sun""s direction vector as a function of time. The use of optical Doppler data in addition to the directional data, though optional for orbit determination, adds a constraint in the dimension perpendicular to that given by the directional data. Inclusion of the optical Doppler data in the orbit determination process can speed up the convergence of the orbit fitting process and improve the orbit solution.
The dual-mode imaging system shown in FIG. 2 is designed for measuring the direction of the Sun. It measures the Sun""s direction using a charge-coupled device camera by capturing the image of the Sun against a background of stars. The stars appearing in the Sun""s image frame serve as a direction reference. The conventional optical imaging system, which is designed for imaging planetary bodies, cannot be directly used for taking the Sun""s image because the Sun is much brighter than the planetary bodies. The design of the invention modifies the conventional optical imaging system by controlling the intensity contrast of light coming from objects with large differences in brightness. The designed image system can take pictures from both planetary bodies and the Sun by operating in two modes: as a regular imager when imaging planetary bodies or as a Sun imager when imaging the Sun.
The navigation system""s use of the Sun as the navigation reference allows the invention to be used in any mission exploring the solar system, and even for missions to nearby stars. The invention can be used for interplanetary space missions, as well as for missions to explore the Sun. Even for Earth satellites, although communication is convenient with Earth-based navigation systems, such as the DSN or the Global Positioning System (GPS), the system of the invention offers an alternative solution as a jam-proof system. Its complete self-containment makes it invulnerable to any destructive interruptions from the ground, such as high impulsive electromagnetic fields, or jamming radio signals. Aside from its main function as a navigation system, this system can also provide other services. Two additional potential applications are for finding the Earth location after a spacecraft xe2x80x9cwakes upxe2x80x9d from a safe mode and for providing spacecraft with high-accuracy Earth direction for optical communication.
The autonomous solar navigation method and apparatus of the invention has demonstrated real space mission feasibility and comparable navigation accuracy with current instrument technology. Given its unique ability, self-containment and universal applicability, benefiting from the technology development and innovation in instrument and sensors, the autonomous solar navigation of the invention has great potential in enhancing spacecraft performance in space exploration.